Yes, that is correct. On my 2009, the horn is rubber mounted so it is insulated from the frame. But cage horns usually are grounded through their mounting bolts especially on American cars. So on your "Beep Beep" horn you may need a little pigtail wire if you retain the rubber insulator on your bike.
Take a look at the attachment PDF. It's for a 2000 Softail but your bike is probably close. The horn shows two wires. The yellow/black wire goes directly to the horn button on the bars. That right there tells you there is no relay and all the horn current flows through the bars horn button. The power source for the horn is the orange/white wire on the other side of the horn button. Note all the other loads that are tapped off that orange wire while it makes it way back to the ACC 15 amp fuse. The ignition switch nomenclature has a type-o in it so don't let that confuse you. If you really have a spare 15 amp fuse as the schematic shows, that would be a perfect place to get power for your cube relay. Pin 30 of the relay would go to that spare 15 amp fuse. Pin 87 of the relay (normally open) would go to the Beep Beep horn. Then a short pigtail from the Beep Beep to frame ground. Check that spare fuse with a meter first to make sure it is hot in RUN position.
The original yellow/black and solid black wires from the factory horn will go to relay pins #85 & #86.
The only caution of the entire project is this..=> Those relays come with and without snubber diodes across the coil winding. It is good practice to have the diode style but chances are you can get away with not having a diode style because your bike is an early model. But if you happen to buy a cube relay with a diode, (also called snubber or suppression diode) you must hook it up with the correct polarity. That's no problem, just point me to the web site where you got the relay and I will tell you what to do with the polarity. It boils down to knowing which pin (85 or 86) goes to the yellow/black wire. Now if you buy a relay without a diode, then don't be concerned with any of this polarity/diode stuff.
Beep Beep!